How much is gsync needed? Technically it isn't needed. If you've a choice between a TN panel and a better quality IPS panel, I'd say go for the IPS monitor regardless of gsync or freesync. Also RRebel makes a good point, if it turns out that you end up with a Vega gpu down the road you'd have a quality IPS monitor which already has adaptive sync (freesync) to use it with.
However, if I were you I'd be weary of the MG279Q. It's an excellent monitor, but it's had a lot of complaints on Amazon and elsewhere with quality control issues. The general consensus on it is to keep RMA'ing it to Asus until you get one that has backlight bleed that isn't too bad, as well as no dead pixels, etc. Note that my experience re the MG279Q if from the holidays a couple years back when I'd considered getting the same model. For all I know, QC may have improved by then.
Though I'd suggest you check out some more IPS monitors that have been released since then, research some reviews, maybe see if you can find something that isn't as pricey, with better quality and QC on sale. Acer, Dell, Vizio, and many, many other manufacturers also create excellent IPS monitors that haven't had the QC issues the Asus MG279Q once (of possibly still?) has. You might even be able to find one with gsync if that suits your needs better. Though again, paying for GSync is a waste imho. Put that extra cash into a monitor that has proven quality rather than having to pay considerably just to have a proprietary tech (GSync) that you likely wont even need or notice much difference using it with a 1080 and a decent 1440p/4k IPS monitor.